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Show 268 ON 'fHE S'rRUCTURE OF 'rHE SKUI1L. pos t en·o r d1' ame t er of wh1'cll bones is in the midd.l.e line, as they are not separated, posteriorly, by the supra-ocCipital, or, ante-riorly, by the frontals. . The orbito-sphenoids are large, and enter Into the com-position of the front wall of the skull. . The alisp~enoids are also large, and contribute to the formation of the s1de walls, as well as of the base, of tb e skull. The squamosal appears in the inte~·io~· of t~1e cra~ium bet'A:ee~ the. parietal, supra-occipital, and penotw, w1th wh1ch last It IS not anchylosed. The periotic, a large and dense ossification, has a very pecu-liar form, being divided into an inner portion, corresponding with the pars petrosa, and an outer thick mass which answers to the tegrnen tympani and pars mastoidea. The tympanic is. a Inere ring of bone, op~n above, and having a thicker anterior than posterior crus. It IS by the former that it is more especially attached to the periotic, though the hinder thinner crus also becomes anchylosed with that bone. The squamosal unites behind and below with the ex-occipital, but leaves a space, superiorly, in which the pars rnastoidea appears on the exterior of the skull. The malar process of the squamosal is exceedingly thick, and extends far forwards as well as transversely outwards. The frontals send very large processes downwards and forwards, as in the Elephant and Tapir, which are not only met by the maxillre, as in the latter animal, but also meet, and indeed are covered by, the nasal processes of the premaxillre. The lachrymals are large, but imperforate. The jugals, thick and curved, are connected with them. rrhe very small nasal bones are fixed by the greater part of their under surfaces to the anterior half of the ethmoid, beyond which they project but little, so that almost the whole of the vast anterior nasal aperture is, in the skeleton, uncovered. rrhe premaxillre are enormous, and constitute a large proportion of the lateral margins of the upper jaw as well as the whole of its anterior region. Their ascending, or nasal, processes are produced forwards instead of downwards, so that the poi~t which corresponds with the spina nasalis anter·ior in Man IS nearly on a level with the top of the head. The alveolar THE SKUJ_JLS OF MAMMALIA. 269 process is even more largely developed, to contain tho incisor tusks of the animal. The maxillre, also large and prolono·ed forwards have thick and long palatine processPs separa 5 ted by ·'a . ~~ry " ' a WI e lllCISlVe foram. en .f rom the prem. axilla· The palati.ne process of t h e palatine 1~ also very tl~Ick, ·but it is shorter than deep, so that the posterwr. nares, ~Inch open behi.nd it, are placed vertically under the lnnd~r part of the anterwr nares, in the dry skull. The vomer, th1ck and stout behind, thin and n· a ge-11·1 { 8 1· n ~ront and abo~e, embraces the lower edge of the ethmoid, ·and IS suturally united to both the palatines and the maxillaries. . The skulls of the Sirenia have resemblances on the one side w1th tho~e of the ungulate Mammals and Proboscidea; on the other, w1th those of the Cetacea, but yet differ in many and 1nost important respects from all. The skulls of the Cetacea present more singular modifications than. th1o se ·o f .a ny other Mammalia. In all these animals , th e ba s~i· - crania axis IS _concave. superiorly, and the primitive separation between the bas~-spheno1d and presphenoid persists for a long time. . The vomer Is very long, and extends backwards on the base of the skull at least as far as the basi-sphenoid, and sometimes covers the whole length of that bone. The ethm~id h~s its ~osterior edge perpendicular, or nearly so, to the basi-cranial axis, and the foramina for the exit of the olfactory nerve are small or obliterated. The frontals enter but very little into the roof of the skul] largely into its anterior and lateral walls. They are prolonged outwards and. forwards into the long and broad supra-orbital processes, wh1ch are concave inferiorly, where they form the roof of the orbital cavity. The parietals. hardl~ appear at all, externally, upon the top of the ~ku1l, th~1r median parts being obscured or interrupted by the Inter-panetal and supra-occipital. They occupy a large space, however, in the temporal fossre. The ex-occipitals and supra-occipitals are enormous. The latt.er, usually increased by coalescence with the large interpanetal, ex tend up to, or beyond, the vertex to 1neet the |